In production lines for assembling a plurality of parts and producing industrial products, there is a work operation of taking out one part, i.e., one workpiece at a time by an industrial robot from a workpiece containing unit in which a number of parts are accommodated and then of assembling it to into a product. For example, when assembling a vehicle body, the vehicle body is produced by jointing a plurality of panel materials using a spot welding method or the like, i.e., the panel materials are assembled into the vehicle body by vacuum-holding the panel materials using a vacuum-sucking pad mounted on a robot arm and by transferring them to the vehicle body from the workpiece containing unit. When the robot arm is operated and the panel material, i.e., the workpiece is vacuum-held by the vacuum-sucking pad, vacuum is supplied to the vacuum-sucking pad under a state where the vacuum-sucking pad applies a predetermined pressing force to the workpiece. For this reason, it becomes necessary to apply the pressing force to the vacuum-sucking pad by the robot arm. In order to exert this pressing force on the vacuum-sucking pad, the vacuum-sucking pad is attached to a reciprocating rod driven by a fluid pressure cylinder and when the vacuum-held workpiece is to be transferred by the robot arm, it becomes necessary to fix the reciprocating rod in order to prevent the workpiece from swinging.
Further, when the vehicle body is assembled, as described in Patent Document 1, spot welding may be performed under a state where fastening between workpieces is carried out by a clamp arm, or the workpiece may be transferred by a carrying truck under a state where the workpiece is fastened by the clamp arm. In the case where the clamp arm is opened/closed by the reciprocating rod of the fluid pressure cylinder, it is necessary to fix the reciprocating rod for opening/closing the clamp arm in order to hold a clamped state for a predetermined period of time.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2003-202004